Memory devices are generally used to store data for computer operations. Because memory is so critical in computer operations, it is generally desirable to use memory devices which store large amounts of data and can be accessed very quickly. One way this can be done is by fabrication of semiconductor devices to store data which can then be accessed by a computer processing device.
One kind of memory device which has proved useful is the electrically programmable read-only memory (EPROM). This is a semiconductor memory which can be "programmed," and which can be accesed by a computer processing device during read operations. EPROM's are particularly attractive in storage applications where non-volatility is desired. Non-volatility is a feature which allows the EPROM's to remember the data stored in the memory device even after power to the memory device is lost. Non-volatility is achieved through the use of "floating-gate" transistors to construct individual data cells in the EPROM memory array. The amount of charge held in the floating gate of an EPROM cell differs depending upon whether the cell is programmed or unprogrammed. Inasmuch as the electrical characteristics of the cell are determined by the amount of charge in the gate, sensing of the cell electrical characteristics can serve as an indication of the programming state of the cell.
When constructing memory devices which have a large number of memory cells, one difficulty which has arisen is that smaller and more closely packed devices are more difficult to fabricate and their characteristics are more difficult to control. Each time a memory array is manufactured, large variations in the memory cell characteristics can result, making it difficult to anticipate the actual current flow through individual memory cells during reading of the cell state.
Therefore, it is an object of the present invention to improve the yield on production of high performance EPROM memory chips by allowing for greater variation in EPROM memory cell characteristics caused by variation in the cell manufacturing process.
It is a second object of the present invention to achieve a fast sensing of the state of an EPROM memory cell.
These and other objects of the present invention will be more clearly understood from an examination of the specification, the drawings, and the accompanying claims.